Transistor amplitude modulator



Feb. 24, 3953 KOROS 2,629,858

TRANSISTOR AMPLTUDE MODULATOR Filed Dec. 29, 1950 227 z3 y MR/ff? h I I V lNvENToR Leske L.Knrns 4d BY ATTORNEY 5 Patented F eb. 24, 1953 Leslie FL. ilioros, Emden, N. il., assigner to `fRadio Corporation-of America, :1t-'corporation 'of Dela.-

' Ware pplicatonDecelnbnM, 1950, :Serial No. 203.13.97

.1 'This invention `relates generally to amplitude modulation systems, 'and particularly relates -to a. vsemi-'conductor ampliler Icircuit "arranged to A modulate the amplitude of a carrier wave.

A semi-'conductor -alnplier circuit maybe tuti# lized forY modulating the Vamplitude rof Aa carrier wave. To this-end, the carrier Awave'maz'r be `iml.

pressed between the emitter and 'base Yelectrodes of ia `semi-'conductoramplier'or transistor. The

rived from the 'output circuit is also'p'ha'se.modu-l lated. "This phase modulation qrepresents dis.- tortion which is `undesired. This vphase distortion ofthe amplitude modulated `output wave is caused by a leakage current between l,the input and output electrodes of the transistor, that is, essentially 'between the "emitter 'and 'collector electrodes. Accordingly,A a portion of v:the car-.- rier wave which is 'the exciting energy `leaks over into 'the output circuit yand-'the 'phase of "this leakage current maybe ydifferent from thepha'se of the amplitude modulated output wave.l The result is 4that the amplitude modulated output wave has a certain amount Jof phase distortion.

'Itis believed that this phase'distortion is caused by the internal impedance `of the transistor. y'Ihi'jsfl internal impedance may cause Athe phase =of the leakage lcurrent Vto Vbe different from Ytllat Nvof the amplitude modulated output wave.

It is accordingly lthe 'principal object c'of the present Iinvention to provide an yimprm'edfeint-- plitude modulation system including a iseinifconf ductor ampli-fiery Awhich 'will substan,tially'elim'ii nate undesired phase modulation loftlfle 4output wave.

A further object of the invention. :is toV provide a `transistor modulator circuit 'arranged-to modu late vthe amplitude of carrier Jwave wherein means are provided vto vcompensate yfor the phase deviating effect of the leakage 4current'"wlhiclf-x is'in herently presen-t in a transistor. I

An amplitude modulation system inaccordance lwith the present invention-comprising '1a semi-conductor Vdevice such as a transistor. s The device includes a `semi-conducting body;V A ibase electrode, an emitter Y'electrode and a f'col'lector electrode `are 'provided lin `contact:with lthe body.

Y a The carrier wave to A'be modulated is impressed between the'emitter and the base. The modulation signal iis applied between the collector and the base. A resonant output circuit is coupled between A"the collector and ba-se and is tuned to the frequency of the Vimpressed 'carrier waveand an amplified carrier Awave amplitude modulated in accordance with the modulation `'signal is 'deriveli from the resonemt -ou'tput circuit.

The semi-conductor device inherently has fa leakage 'current bet-Ween emitter and collector which will 4produce an Aundesired phase modulation of the amplitude modulated carrier wave because-tina leakage current Ymay ybe out of phase with the "amplified ywaive which is `delivered by the collector circuit. In accordance with -the present invention a A predetermined portion of the 'modulationsignal is impressed on Vthe emitter Aandbase 'electrodes simultaneously -with the carv Arier wave, whichwis 4to be `ampliiied and amplitude` modulated in the collector circuit. Consequently, "the vcarrier wave energy impressed on the emitteri'of the semi-conductor device is also modulated I'to "a predetermined extent and in a predetermined direction. In this manner the undesired phase ymodulation of the amplitude modulated output carrier wave is substantially eliminated.

"Other amplitude modulation systems which will compensate for the lundesired phase distortion of `the amplitude modulated-output wave `inherent in .the 'transistor have been disclosed and claimed inthe copending application to L. `L. Kores en'- trtled "Semi-Conductor Amplitude Modulation Systems and led concurrently herewith.

"The novel featuresL that are considered 'characteristic "of this invention are set -forth with particularly in the Aappended claims. The in'- venti'on itself, however, both as to yits organization --and method of operation, -as well as additional `objects and advantages thereof, will lbest be yunderstood from the `following description when read in connection withthe accompanying drawing, fi-n which:

Figure =1 i-s a circuit diagram of an amplitude modulation system embodying the present invention.

Figures '2 and 3 are v-ve'ctor diagrams which will be -ref'er-redto in "explaining fthe operation of the circuit'ofFigure 1; and

Figure `4 isa graph illustrating -the 'amplitude modulated exciter carrierwave as applied to the emitter in the ci-rcuit'of Figure l1.

Relerringnow to AFigure 1, lthere is Villustrated anamplitude modulation system comprising a semi-conductor device II). The device I includes a semi-conducting body II which may, for example, consist of silicon or preferably of germanium. The semi-conducting body II is prepared in a conventional manner well known in the art. A base electrode I2, an emitter electrode I3 and a collector electrode I4 are in contact with body II. Base electrode I2 is in lowresistance contact with the body II and may be a. large-area electrode as indicated. Emitter electrode I3 and collector electrode I4 are in rectifying contact with the body II; they may be point electrodes as indicated or they may be in line contact with body II or even in large-area contact provided they form rectifying contacts with the body.

Operating potentials are applied to the electrodes I2 to I4. As is well known a potential in reverse direction is applied between collector I4 and base I2. Furthermore, a potential in the forward direction is applied between emitter I3 and base I2. If semi-conducting body II is of the N type, the collector I4 should be negative and the emitter I3 positive with respect to the base I2. If the body II should be of the P type, the potentials must be reversed. It will be assumed for the following discussion that body II is of the N type.

Base electrode I2 may be grounded as shown. and a source of potential such as battery I5 may have its positive terminal grounded while its negative terminal is connected through in. ductor I6 and parallel resonant circuit I1 to collector I4. The emitter electrode may also be biased by a suitable battery; however, it is I.

feasible as shown in the drawing to provide instead a bias network I8 including resistor 20 and capacitor 2I connected in parallel. One terminal of bias network I8 is grounded as shown while its other terminal is connected to emitter I3 in a manner to be described hereinafter.

A source of a carrier wave is indicated at 22.Y The carrier wave is impressed on emitter I3 and may have any frequency provided it is'higher than the highest frequency of the modulation signal. Furthermore, the carrier wave developed by source 22 may be a sinusoidal wave or a wave of any other suitable shape. One terminal of carrier wave source 22 is coupled to emitter elec' trode I3 by a coupling capacitor 23. A choke coil 24 is connected between emitter I3 and the other terminal of carrier wave source 22. The lower terminal of choke coil 24 is grounded through capacitor 25 which bypasses carrier frequency currents.

A source of a modulation signal is indicated at 26. The modulation signal may, for example, be an audio signal or a video signal or any other type of signal with which it is desired to modulate the carrier wave. Modulation signal source 26 is connected across inductor 21 forming the pri--A mary winding of a transformer, the secondary Winding of which is formed by inductor I6. Transformer I6, 21 may be provided with a core'` as indicated provided the modulation signal is an audio signal. v former I6, 21 may be connected through coupling capacitor 28 while the lower terminals of the transformer may be coupled throughlca.-v pacitor 30. Capacitors 28, 30 are provided' to' correct the phase relation between the primary winding 21 and the secondary winding I6 so that. the modulation signal is impressedl on the sec-: ondary Winding with the proper phase. Capacitors 26 and 30 must be connectedbetweenthe 'I'he upper terminals of trans-4 anglev'a withthe output vector` AC. The vector,

4 windings when the transformer windings have a 1: 1 turn ratio.

Battery I5 may be bypassed for modulation signal currents by capacitor 3I. Capacitor 32 is provided between the junction point of secondary winding I6 and parallel resonant circuit I1, on the one hand, and ground, on the other hand, to bypass carrier frequency currents. Capacitor 2I of bias network `I 8 is arranged to bypass modulation signal currents. Bias network I8 is connected to emitter electrode VI3 through the lower portion of primary winding 21, variable tap 33 on conductor 31, lead 34 and choke coil 24,

Parallel resonant circuit I1 is preferably tuned to the frequency of the input carrier wave, and an output load impedance element such as resistor '36 is connected across parallel resonant circuit I1. Load resistor 36 may represent a utilization circuit such as the input circuit of a following amplifier stage. The output signal may be obtained from output terminals 31, in which case resistor 36 may be omitted or its resistance properly increased.

Disregarding for the present the connection throughlead 34 between modulation signal source 26 and emitter electrode I3, the circuit of Figurev l operates in a conventional manner. The carrier wave developed by source 22 is effectively impressed between emitter I3 and base I2. Accordingly, an amplified version of the carrier wave Aappears atY the output terminals 31. It is well known that emitter I3 and base I2 operate as a peak rectier of the impressed carrier wave. The rectified direct current ows through choke coil 24, lead 34, tap 33, the lower portion of primary winding 21 and bias network I8 to ground, that is, to base I2. The rectified current eventually builds up a charge across capacitor 2|'Which biases the emitter electrode I3. The thus developed bias voltage preferably is of such a magnitude as to provide class C operation of the semi-conductor device I0.

The collector bias voltage supplied by battery I5 is modulated by the modulation signal source 26 .in accordance with the modulation signal.- Accordingly, the amplified output wave which may be obtained from output terminals 31 has its amplitude modulated in accordance with the modulation signal.

However, this amplitude modulated output wave also may have an undesired phase modula' tion. As explained hereinbefore, this phase modulation is due to a leakage current which is inherently present in the device I0 and which flows essentially between emitter I3 and collector I4. Since this leakage current may be out of phase with the amplitude modulated output wave, phase modulation is produced. The effect of thisv leakage current may be more readily understood by reference to the vector diagram of Figure 2.

p The voltage vector AB in Figure 2 represents the unmodulated output carrier wave which is developed at load impedance element 36. The voltage vector BC is the vector sum of the rotating. s ide bands which are produced by the amplitude modulation during peak modulation of the carrier wave. Consequently vector AB equals vector BC for per cent modulation. The vector AD representsthe leakage voltage which forms an AE represents the output carrier wave volt, aigeaif no modulation is present. If the leakage vector vAD'is added to the modulated carrier wave` vector AC.' the peak envelope voltage is shown by accepts .5 the voltage vector It fw'ill be seen that 'an angle m is formed between the vector and AF. This angle m -is a ffunction of the .modulation signal. Accordingly, the Aoutput carrier wave, which is produced by a combination of the vvectors. AD and AC, has fa phase modulation, the angle -m lof which varies during the modulation cycle.

In accordance withv the presentzinvention the leakage voltage vector ADis also modulated with the modulation signal thereby tovf'eliminateithe -eifect of the phase distortion. This'effect .has been illustra-ted in vlligure I3 where the vectors AB, BC, AD and 'represent thesam'e voltages as in Figure 2. Y'If the leakage voltage vector AD is modulated in the same ratio as the output Wave, it becomes AD'. Consequently, the vector sum AD and AB and the vector `sumfilll and AC form a resulting vector .AE-'or AF which forma straight line softhat `the angle im :becomes zero. Regardless of the modulation level of the carrier wave, the resulting vector AE or AF' will bedisposed in a straight line -so that no phasemodulation is present.

ln accordance with the `present invention, Athis is effected by impressing asportion Aof the modulation signa-l through adjustable 'tap 33,' 'lead-34, and choke coil 24 on-emitter lelectrode I3.` Consequently both the carrier wave impressed fon device I and -theamplined-carrier wave developed in resonant circuit I 1 --are modulated simultaneously by the modulation signal.

IThe procedure 'for determining the percentage of the modulation with kwhich the impressed input carrier wave is to "be modulated will 'bev explained in connection with Figure 4. Figure `4 illustrates schematically the exciter carrier wave which is applied to the yemitter and vbase electrodes from the source '22. 'This wave will be amplified and amplitude modulated Vin 4accordance with the modulation signal andthe loutput wave will be nally delivered to load resistor 36. The envelope of the carrier wave'premodulated according to this invention Vis indicated at 4l). The carrier wave below horizontal "line 4I represents the 'unloaded'p'ortion of the exciter'voltage which exhibits a higher amplitude than theiupper loaded portion ofthe wave because the carrier wave sufers a `voltage drop `due to the internal resistance of source 22 and of device I0.

The emitter -bias voltage is indicated by a dotted line 42. Dotted line 43 indicates :the peaks of the unmodulated exciter carrier wave, that is, in the absence of the additional modulation signal in the emitter circuit. Dotted line AHl .indicates the modulation peaks of the exciter voltage. The emitter 'bias voltage 42 effectively makes inactive the unmodulated part of the impressed carrier Wave which appears below line 42. The applied carrier wave preferably is modulated up to the emitter bias shown at 42. The acting or active part of the carrier wave is the portion above the emitter bias line 42. Preferably the active portion of the carrier Wave is modulated close to 100 per cent.

The modulation voltage applied to the emitter must, of course, be in phase with the modulation voltage applied to the collector I4. To this end, the leakage between tap 33 and the lower terminal of inductor 2l, and the entire inductor 21 should be as small as possible. Under these conditions the component of the leakage current is modulated in the same ratio as the applied carrier wave as illustrated in Figure 3 at any modulating frequency.

By way of example 'in a circuit in accordance k.with the present invention lthe voltage of the applied carrier wave .excitationzbetween .the emitter and ground is 2.25 volts R. M. S. rIIhe .modulation signal lapplied yto the collector I4 is 2.45 volts 1S. :in order -to produce 1.00 per cen-t amplitude modulation Without `phase .modulation provided the .2.25wvolts R. M. S. excitation 4is 'also modulated 'as `described here. The emitter bias vcltagefis 2.6 volts D. C. corresponding to dotted line 'i2-.in .Figure .4. Under 'these conditions the modulation signal applied `to emitter I3 has a voltage of v.4'3 lvolt yR. M. lS. Ifthe impressed carrier wave is unmodulated, 'the acting Vpeaks of the wave 'has a voltage of 2.25 \/2-2.6=;5'8 'volt R. M. S. corresponding to the distance between dotted lines4`3 and l42 in Figure 4. The theoretical R. 4M. S. voltagerequired to modulate the 'carrierinput wave 'to prevent .phase modulation 'was found. to be .58/\/2=.41 volt R. M. S. which is in close agreement with the experimentally observed value of ..43 volt R. M. S.

There .has .thus been disclosed an amplitude modulation System including a transistor which will -substantially eliminate .the undesired phase distortion due to `the leakage `current inherent in a transistor. This is leffected by applyinga portion of the modulationsig-nalto the input elec-.- trodes ofthe device.

It hasv previously been proposed to modulate the exciting power of plate .modulated electron tubes. However', in that case, the additional grid modulation was proposed to ,produce a more nearly .straight line rela-tion between the modulating Ainput radio frequency voltages and the resulting output radio frequency tank voltages. Furthermore, the .amount of the additional grid modulation utilized in a plate modulated electron tube is different from the amount of modulation applied to theinput electrodes of a transistor in accord-ance with the present invention. The input electrodes of the transistor are modulated by the modulation signal .only for the purpose Aof eliminating phase distortion ofthe amplitude modulated wave, which is not thepurpose of the grid modulation referred to.

l. An `amplitude modulation -system comprising va lserali-conductor :device .including :a semiconducting body, a base electrode, an emitter electrode and a collector electrode in contact with said body, means applying operating. potentials to said electrodes, a vcarrier wave source coupled betweeny said emitter `and base electrodes `for im pressing .said carrier wave thereon, a modulation signal source coupled between said collector and base electrodes for impressing said modulation signal thereon, a resonant output circuit coupled between said collector and base electrodes and tuned to the frequency of said carrier wave for deriving therefrom an amplified carrier wave amplitude modulated in accordance with said modulation signal, and a circuit connection between said signal source and said emitter and base electrodes for impressing a predetermined portion of said modulation signal on said emitter and base electrodes simultaneously with said carrier wave, thereby to modulate said carrier wave impressed on said device to a predetermined extent and in a predetermined direction as provided by said modulation signal.

2. An amplitude modulation system comprising a semi-conductor device including a semi-conducting body, a base electrode, an emitter electrode and a collector electrode in contact with said body, means applying a potential in the reverse direction between said collector and base electrodes and a potential in the forward direction between said emitter and base electrodes, a carrier wave source coupled between said emitter and base electrodes for impressing said carrier wave thereon, said impressed carrier wave having an active portion exceeding said potential in the forward direction, a modulation signal source coupled between said collector and base electrodes for impressing said modulation signal thereon, a resonant output circuit coupled between said collector and base electrodes and tuned to the frequency of said carrier wave for deriving therefrom an amplified carrier wave amplitude modulated in accordance with said modulation signal, and a circuit connection between said signal source and said emitter and base electrodes for impressing such a portion of said modulation signal on said emitter and base electrodes simultaneously with said carrier wave as to modulate said active portion of said carrier wave impressed on said device substantially to the same extent as said amplied carrier wave is modulated by said modulation signal.

3. An amplitude modulation system comprising a semi-conductor device including a semi-conducting body, a base electrode, an emitter electrode and a collector electrode in contact with said body, means applying a potential in the reverse direction between said collector and base electrodes, and a potential in the forward direction between said emitter and base electrodes, a carrier wave source coupled between said emitter and base electrodes for impressing said carrier wave thereon, a modulation signal source coupled between said collector and base electrodes for impressing said modulation signal thereon, a resonant output circuit coupled between said collector and base electrodes and tuned to the frequency of .said carrier wave for deriving therefrom an amplied carrier wave amplitude modulated in accordance with said modulation signal, and a circuit connection between said signal source and said emitter and base electrodes for impressing such a portion of said modulation signal on said emitter and base electrodes simultaneously with said carrier wave as to modulate substantially completely the portion of said carrier wave impressed on said device which exceeds said potential in the forward direction.

4. An amplitude modulation system comprising a semi-conductor device including a semi-conducting body, a base electrode, an emitter electrode and a collector electrode in contact with said body, means connecting said base electrode to ground, a carrier wave source,a modulation signal source, a parallel resonant output circuit tuned to the frequency of said carrier wave, a transformer having a primary winding connected across said modulation signal source and a secondary winding, a source of voltage, said output circuit, said secondary winding and said source of voltage being connected serially between said collector electrode and ground, said source of voltage being so poled as to apply a voltage in the reverse direction between said collector and base electrodes, said carrier wave source being coupled to said emitter electrode, an inductor arranged as a carrier wave choke and connected across said carrier wave source and between said emitter electrode and an intermediate point of said primary winding, and means connected between said emitter electrodes and ground applying a bias voltage to said emitter electrode, whereby a predetermined portion of said modulation signal is applied to said emitter electrode.

5. An amplitude modulation system comprising a semi-conductor device including a semi-conducting body, a base electrode, an emitter electrode and a collector electrode in contact with said body, means connecting said base electrode to ground, a carrier wave source, a modulation signal source, a parallel resonant output circuit tuned to the frequency of said carrier wave, a transformer Ahaving a primary winding connected across said modulation signal source and a secondary winding, a direct current source of operating voltage connected serially with said output circuit and said secondary winding between said collector electrode and ground, said source of voltage being so poled as to apply a voltage in the reverse direction between said collector and base electrodes, said carrier wave source being coupled to said emitter electrode, an inductor arranged as a carrier wave choke and connected across said carrier wave source and between said emitter electrode and an intermediate point of said primary winding, and a resistor and a capacitor connected in parallel between one terminal of said primary winding and ground to develop a bias voltage for said emitter electrode, whereby a predetermined portion of said modulation signal is applied to said emitter electrode.

LESLIE L. KOROS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES' PATENTS Number Name Date 2,436,066 Favre Feb. 17, 1948 2,486,776 Barney Nov. l, 1949 

